The present invention relates to a centerless grinder which comprises a grinding wheel spindle stock and a regulating wheel spindle stock disposed in opposed relation, and a work rest disposed between the grinding wheel spindle stock and the regulating wheel spindle stock.
One exemplary centerless grinder of the aforesaid type is shown in FIG. 9. FIG. 9 is a front view illustrating the centerless grinder partly in section. As shown in FIG. 9, the centerless grinder 100 includes a bed 108, a grinding wheel spindle stock 101 disposed on the bed 108 and having a grinding wheel 101a, a regulating wheel spindle stock 102 disposed in opposed relation to the grinding wheel spindle stock 101 and having a regulating wheel 102a, a work rest 103 disposed between the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 and having a blade 103a for supporting a workpiece W, feed devices 104, 104 for moving the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102, and the like. By the operations of the feed devices 104, 104, the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 are moved toward and away from each other.
The feed devices 104, 104 include feed screws 105, 105 respectively disposed as extending in the directions of the movements of the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102, driving motors 106, 106 for respectively rotating the feed screws 105, 105 about axes thereof, nuts 107, 107 respectively fixed to the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 in threading engagement with the feed screws 105, 105, and the like.
Brackets 109, 109 fixed to opposite sides of the bed 108 respectively hold the feed screws 105, 105 so as to restrict the axial movements thereof and permit the rotational movements thereof about the axes thereof. The feed screws 105, 105 are respectively rotated by the driving motors 106, 106, whereby the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 are moved toward and away from each other.
Thus, the feed devices 104, 104 are respectively driven to properly move the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102, so that the workpiece W is supported by the grinding wheel spindle stock 101, the regulating wheel spindle stock 102 and the work rest 103, and the workpiece W has a predetermined depth of grinding. Then the workpiece W is thrown between the grinding wheel 101a and the regulating wheel 102a which are now rotating. Thus, the workpiece W is ground by the grinding wheel 101a. 
When the workpiece W is ground by the centerless grinder 100, a grinding liquid is supplied to a grinding part so as to cool the grinding part. Heat generated during the grinding is absorbed by the grinding liquid to increase the temperature of the grinding liquid. Where the temperature of the grinding liquid is 20xc2x0 C. immediately after the start of the machining, for example, the grinding liquid is heated up to about 35xc2x0 C. to about 40xc2x0 C. after a lapse of 20 to 30 minutes from the start of the machining.
The grinding liquid supplied to the grinding part generally drops on the bed 108, and then is collected in a storage tank. Therefore, the temperature of an upper portion of the bed 108 is increased by the grinding liquid having dropped on the bed 108. On the other hand, a lower portion of the bed 108 is less susceptible to the increase in the temperature of the grinding liquid. Hence, the upper portion of the bed 108 is more liable to be thermally deformed than the lower portion of the bed 108 as shown in FIG. 10. In FIG. 10, the thermal deformation of the bed 108 is exaggerated for purpose of illustration, and an initial state immediately after the machining and a thermally deformed state are indicated by a continuous line and a chain double-dashed line, respectively.
As shown, the upper portion of the bed 108 is thermally deformed to be convexly curved upward. Since the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 disposed on the bed 108 are upwardly offset and the feed screws 105 are held on the opposite sides of the bed 108, the grinding wheel spindle stock 101 and the regulating wheel spindle stock 102 are displaced away from each other by the thermal expansion of the upper portion of the bed 108. Therefore, the centerless grinder 100 having the aforesaid construction conventionally suffers from a problem such that the grinding dimensional accuracy is remarkably reduced by the thermal deformation.
In view of the foregoing, it is an object of the present invention to provide a centerless grinder which can minimize the reduction in machining accuracy even if the bed is thermally deformed by the heat generated during the grinding.
In accordance with the present invention, there is provided a centerless grinder, which comprises: a bed; a grinding wheel spindle stock and a regulating wheel spindle stock disposed on the bed in opposed relation; a work rest fixed on the bed between the grinding wheel spindle stock and the regulating wheel spindle stock for supporting a workpiece between the grinding wheel spindle stock and the regulating wheel spindle stock; at least one of the grinding wheel spindle stock and the regulating wheel spindle stock serving as a feedable wheel spindle stock which is movable in a direction toward and away from the other wheel spindle stock; and a feed device for moving the feedable wheel spindle stock in the direction, wherein the feed device comprises: a feed screw having one end fixed onto the bed between the grinding wheel spindle stock and the regulating wheel spindle stock, and disposed as extending in the direction of the movement; a nut retained in the feedable wheel spindle stock in threading engagement with the feed screw so that an axial movement thereof along the feed screw is restricted and a rotational movement thereof about an axis thereof is permitted; and a driving motor for rotating the nut about the axis thereof.
In the centerless grinder, the one end of the feed screw is fixed onto the bed and, when the nut is rotated about the axis thereof by the driving motor, the nut threadingly engaged with the feed screw and the feedable wheel spindle stock retaining the nut are moved along the feed screw. The one end of the feed screw is thus fixed onto the bed between the grinding wheel spindle stock and the regulating wheel spindle stock and, therefore, even if the bed is thermally expanded by heat generated during the grinding, the nut threadingly engaged with the feed screw and the feedable wheel spindle stock retaining the nut are kept in a proper positional relationship with respect to the feed screw by the threading engagement with the feed screw irrespective of the thermal expansion of the bed. As a result, the positional relationship between the grinding wheel spindle stock and the regulating wheel spindle stock is properly maintained.
The inventive centerless grinder is free from a significant change in the positional relationship between the grinding wheel spindle stock and the regulating wheel spindle stock, which often occurs due to the thermal deformation of the bed in the conventional centerless grinder in which the feed screws are retained on the sides of the bed. Therefore, the inventive centerless grinder provides an effect of minimizing the reduction in the machining accuracy of the workpiece which may occur due to the thermal deformation of the bed.
The centerless grinder may be constructed such that the grinding wheel spindle stock and the regulating wheel spindle stock respectively serve as feedable wheel spindle stocks which are movable in a direction toward and away from each other, wherein the feed device comprises: a single feed screw disposed as extending in the direction of the movements of the feedable wheel spindle stocks and having a middle portion fixed onto the bed between the grinding wheel spindle stock and the regulating wheel spindle stock; a first nut retained in the grinding wheel spindle stock in threading engagement with one side of the feed screw so that an axial movement thereof along the feed screw is restricted and a rotational movement thereof about an axis thereof is permitted; a second nut retained in the regulating wheel spindle stock in threading engagement with the other side of the feed screw so that an axial movement thereof along the feed screw is restricted and a rotational movement thereof about an axis thereof is permitted; a first driving motor for rotating the first nut about the axis thereof; and a second driving motor for rotating the second nut about the axis thereof.
In the centerless grinder, the middle portion of the feed screw is retained on the bed between the grinding wheel spindle stock and the regulating wheel spindle stock and, therefore, when the first nut and the second nut are respectively rotated about the axes thereof by the first driving motor and the second driving motor, the first nut and the second nut threadingly engaged with the feed screw and the grinding wheel spindle stock and the regulating wheel spindle stock respectively retaining the first nut and the second nut are moved along the feed screw.
The single feed screw is retained on the bed between the grinding wheel spindle stock and the regulating wheel spindle stock as described above and, therefore, even if the bed is thermally expanded by heat generated during the grinding, the first nut and the second nut threadingly engaged with the feed screw and the grinding wheel spindle stock and the regulating wheel spindle stock respectively retaining the first nut and the second nut are kept in proper positional relationships with respect to the feed screw by the threading engagement with the feed screw irrespective of the thermal expansion of the bed. As a result, the positional relationship between the grinding wheel spindle stock and the regulating wheel spindle stock is properly maintained. Thus, the reduction in the machining accuracy of the workpiece can be minimized which may occur due to the thermal deformation of the bed.
The feed screw may be fixed to the work rest, a retainer provided on the bed as a part thereof or an additional component fixed onto the bed.
The single feed screw may be retained in a slightly axially and/or radially movable manner by the retainer, the work rest or the additional component fixed onto the bed.
In general, the grinding wheel spindle stock and the regulating wheel spindle stock are guided by proper guiding means so as to be movable toward and away from each other. Where the grinding wheel spindle stock and the regulating wheel spindle stock are adapted to be moved by the single feed screw as described above and the feed screw is fixed onto the bed, it would be necessary to precisely align the guide means for guiding the grinding wheel spindle stock and the regulating wheel spindle stock with the first nut and the second nut respectively retained in the grinding wheel spindle stock and the regulating wheel spindle stock. Without proper alignment, the grinding wheel spindle stock and the regulating wheel spindle stock could not smoothly be moved. As a result, the feed screw, the first nut and the second nut, or the guide means would abnormally be worn, thereby presenting a problem such as deterioration in grinding accuracy. Further, the precise alignment of the guide means with the first nut and the second nut would be troublesome, and adjustment for the alignment would be time-consuming.
In the centerless grinder having the aforesaid construction, the feed screw is allowed for slight axial and/or radial movement. Even if the guide means is slightly misaligned with the first nut and the second nut, the misalignment is accommodated by the slight movement. Therefore, the grinding wheel spindle stock and the regulating wheel spindle stock can smoothly be moved without the precise alignment of the guide means with the first nut and the second nut. Even if the bed is thermally deformed to be curved, the deformation is accommodated by the slight movement, so that deformation of the feed screw can be prevented which may otherwise occur due to the thermal deformation of the bed.
For the retention of the feed screw, the feed screw has a flange formed around the middle portion thereof and having at least two through-holes formed therein as extending therethrough axially of the feed screw, and tubular sleeves each having a length greater than the thickness of the flange are respectively loosely fitted in the through-holes and fixed to the retainer or the work rest by fixing bolts. With this relatively simple arrangement, the feed screw is allowed for slight axial and radial movements.
The first nut and the second nut may respectively be retained in the grinding wheel spindle stock and the regulating wheel spindle stock via thrust bearings each having an aligning function. With this arrangement, a load exerted on the feed screw is alleviated by the aligning function, even if the feed screw is misaligned with the thrust bearings. As a result, the feed screw is prevented from being bent or deformed.
Preferably, the driving motors are respectively fixed to the feedable wheel spindle stocks.
The feed screw is preferably composed of a material having a coefficient of linear expansion smaller than that of the bed. If the feed screw is thermally expanded, the thermal expansion directly influences a spacing between the grinding wheel spindle stock and the regulating wheel spindle stock, so that the finished workpiece may have a dimensional error. Therefore, the feed screw is preferably composed of a material having a smallest possible coefficient of linear expansion, and desirably composed of a material having a coefficient of linear expansion smaller than at least that of the bed. To this end, it is further preferred that the feed screw is composed of a material having a coefficient of linear expansion of not greater than 4.0xc3x9710xe2x88x926/xc2x0 C.